Tumors that start in the brain (primary brain tumors) are not the same as tumors that start in other organs, such as the lung or breast, and then spread to the brain (metastatic or secondary brain tumors). In adults, metastatic tumors to the brain are actually more common than primary brain tumors. These tumors are not treated the same way. For example, breast or lung cancers that spread to the brain are treated differently from tumors that start in the brain.
Primary brain tumors can start in almost any type of tissue or cell in the brain or spinal cord. Some tumors have a mixture of cell types.
- Gliomas: Gliomas are not a specific type of brain tumor. Glioma is a general term for a group of tumors that start in glial cells. A number of tumors can be considered gliomas, including glioblastoma (also known as glioblastoma multiforme), astrocytomas, oligodendrogliomas, and ependymomas. About 3 out of 10 of all brain tumors are gliomas. Most fast-growing brain tumors are gliomas.
- Astrocytomas: Astrocytomas are tumors that start in glial cells called astrocytes. About 2 out of 10 brain tumors are astrocytomas. Most astrocytomas can spread widely throughout the brain and blend with the normal brain tissue, which can make them very hard to remove by surgery.
- Oligodendrogliomas: These tumors start in brain glial cells called oligodendrocytes. These tumors tend to grow slowly, but like astrocytomas, most of them can grow into (infiltrate) nearby brain tissue and cannot be removed completely by surgery. Oligodendrogliomas sometimes spread along the CSF pathways but rarely spread outside the brain or spinal cord. As with astrocytomas, they can become more aggressive over time. Very aggressive forms of these tumors are known as anaplastic oligodendrogliomas. Only about 2% of brain tumors are oligodendrogliomas.
- Ependymomas: These tumors arise from ependymal cells, which line the ventricles. They can range from fairly low-grade (less aggressive) tumors to higher grade ones, which are called anaplastic ependymomas. Only about 2% of brain tumors are ependymomas. Ependymomas are more likely to spread along the CSF pathways than other gliomas but do not spread outside the brain or spinal cord. Ependymomas may block the exit of CSF from the ventricles, causing the ventricles to become very large – a condition called hydrocephalus.
- Meningiomas: Meningiomas begin in the meninges, the layers of tissue that surround the outer part of the brain and spinal cord. Meningiomas account for about 1 out of 3 primary brain and spinal cord tumors. The risk of these tumors increases with age. They occur about twice as often in women.
- Medulloblastomas: Medulloblastomas develop from neuroectodermal cells (primitive nerve cells) in the cerebellum. They are fast-growing tumors and often spread throughout the CSF pathways, but they can be treated by surgery, radiation therapy, and chemotherapy. Medulloblastomas occur much more often in children than in adults. They are part of a class of tumors called primitive neuroectodermal tumors (PNETs) that can also start in other parts of the central nervous system.
- Schwannomas (neurilemmomas): Schwannomas develop from Schwann cells, which surround and insulate cranial nerves and other nerves. They make up about 8% of all CNS tumors. Schwannomas are almost always benign tumors. They can also start on spinal nerves after the point where they have left the spinal cord. When this happens, they can press on the spinal cord, causing weakness, sensory loss, and bowel and bladder problems.
- Chordomas: These rare tumors start in the bone at the base of the skull or at the lower end of the spine. Chordomas don’t start in the central nervous system, but they can injure the nearby brain or spinal cord by pressing on it.
- Pituitary tumors: Tumors that start in the pituitary gland are almost always benign (non-cancerous). But they can still cause problems if they grow large enough to press on nearby structures or if they make too much of any kind of hormone.